1. Field of the Invention
This invention relates to the repair of fluid transporting pipelines such as water, gas, oil or other pipelines for civil and industrial use.
2. Discussion of the Related Art
These pipelines are generally formed by welding together lengths of metal pipe, the continuous pipeline assembled in this manner being laid in its final seat, generally consisting of a sufficiently deep trench, and then covered in order to recreate the pre-existing environment and hence not hinder human activity on its surface.
The assembled continuous pipeline is provided with protection against wet corrosion, as the environment in which the underground pipeline lies is very aggressive towards ferrous materials, this protection generally being in the form of protective coverings and cathodic protection with an applied current, and being provided with a monitoring circuit and frequently inspected. The integrity and preservation of the pipeline during its entire operating life are very important, not only because of its high construction cost, but also to prevent danger, pollution or serious disturbance to users.
This protection is often insufficient to protect an underground pipeline for the required time for the following reasons. The covering materials are not free of a certain porosity and a certain degradation with time, and during the application of the covering and the laying and burying of the pipeline slight damage to and/or separation of the covering can occur, and which can subsequently trigger corrosion phenomena even after a considerable time. Natural phenomena, such as earthquakes, collapsing and landslips, or accidental events deriving from human activity on the surface, can cause local damage to the pipeline. Various methods for repairing the damaged portions have been proposed and practiced in the known art.
The most radical repair method consists of replacing the damaged portion by removing the damaged metal pipe piece and inserting a new pipe piece in its place, so basically repeating the pipeline construction procedure. This method has considerable drawbacks because it is very difficult and costly, and because it puts the entire pipeline out of operation for a prolonged time. In a less radical but rather questionable method it has been proposed to make the repair by applying a reinforcement to the outside by welding on a piece of metal plate of the same shape as the damaged part or by directly inserting new material into the damaged region to restore the thickness of the pipeline.
Another handy repair method of more widespread use is to apply two cylindrical steel half-shells to the outside of the damaged portion of the pipeline and weld them together along their generators, to hence create a metal shell about the damaged portion. The interspace between the pipeline and the two half-shells, these being known as cladding, is sealed at its two ends with a cement mix. After waiting for the cement mix to set, a resin comprising two or more components is injected into the interspace of circular ring cross-section created between the pipeline and the welded cladding, and is allowed to harden within the interspace to produce a sheath which creates continuity between the inner damaged pipe piece and the outer shell, so that the force due to the pressure within the pipeline is discharged onto the shell.
This method has considerable drawbacks due both to the waiting time for the cement to set and to the fact that having used a cement mix for sealing, the resin components cannot be injected at high pressure. If the resin were to be injected at high pressure, the cement seal would not be able to remain in position and would be pushed outwards to allow the resin to escape. Injecting the resin at insufficient pressure can result in incomplete filling of the interspace and in addition this method cannot be relied on to produce continuity between the damaged pipe piece and the outer shell because of the fact that resins generally shrink during polymerization and hardening, even if inert fillers have been added, with the result that the force deriving from the pressure within the pipeline is not completely discharged onto the shell. This discontinuity between the shell and resin sheath is due to this shrinkage phenomenon.